Estimating ages of late Quaternary stream terraces from analysis of weathering rinds and soils PETER L. K. KNUEPFER Department of Geological Sciences and Environmental Studies, State University of New York at Binghamton, Binghamton, New York 13901 ABSTRACT Rates of growth of rock weathering rinds and rates of changes of soil morphologic and chemical properties that are calibrated at sites of known ages produce powerful tools for estimating ages of geomorphic surfaces, par- ticularly when materials datable by conven- tional methods are not widespread. Se- quences of latest Quaternary stream terraces from the South Island of New Zealand are ideally suited for application of these cali- brated-age techniques because climate and source materials for terrace deposits are in- ternally consistent within two regions, West- land and Marlborough. Modal and mean thicknesses of weathering rinds formed in surface cobbles of Torlesse graywacke pro- vide age estimates for latest Pleistocene and Holocene stream terraces in Marlborough with uncertainties estimated at ±5% to ±40%. Modal rind thickness of subsurface cobbles provides age estimates for latest Quaternary Westland sites with uncertainties of ±10% to more than ±50%. Few dated soils are availa- ble to constrain rates of soil development in Westland and Marlborough. Furthermore, soils were sampled from only one pedon on each surface. Morphologic data, however, provide least-squares age estimates for West- land sites with uncertainties typically greater than ±100%, and the calibration data can further bracket likely soil ages to less than 50% uncertainty. The regression data from Marlborough are less reliable, but profile- index ratios help bracket ages to at least ±50%. Distributions of oxalate-extractable Fe and A1 and total major-element concentra- tions allow age estimates for Westland and Marlborough soils with uncertainties esti- mated at ±15% to ±50%. Despite these uncer- tainties, the three techniques combine to provide age estimates with the high resolution needed to examine details of fault-slip rates or stream downcutting. The calibration data and weathering rates used herein are applicable only to the New Zealand region studied, al- though similar calibrated-age techniques should provide high-resolution dating in many other regions. Such techniques are par- ticularly useful when multiple approaches are available to provide independent cross- checks. INTRODUCTION Geomorphic surfaces such as alluvial fans,  stream terraces, marine terraces, and moraines  form reference features that can be used to infer  rates of tectonic processes such as faulting or  uplift and rates of geomorphic processes such as  river downcutting. In these applications, it is es- sential that the age of the geomorphic surface, rather than the age of the underlying deposits, be  known. The conventional approach to dating  late Quaternary geomorphic surfaces is to obtain  Figure 1. Location maps of calibration and study sites, (a) Calibration sites. Rind calibration sites are numbered and keyed to Table 1. Soil sites are lettered: A. Franz Josef, B. Okuru River, C. Buller River, D. Ben Ohau, E. Charwell River, (b) Soil and weathering-rind study sites. Weathering-rind sites are numbered and keyed to Table 2. Soil study sites are lettered: F. Wanganui River, G. Moeraki River, H. Mahitahi River, I. Toaroha River, J. Branch River, K. Saxton River, L. Grey River, M. Edwards-Waiau River, N. Clarence River, O. Charwell River. Additional material (appendices) for this paper may be obtained free of charge by requesting Supplementary Data 8816 from the GSA Documents Secretary. Geological Society of America Bulletin, v. 100, p. 1224-1236,10 figs., 4 tables, August 1988. 1224